CHICAGO — About half of America’s migratory birds fly from destinations as far-flung as Chile to nest in Canada’s boreal forest. In Alberta, that forest lies above tar sands that contain oil reserves second only to Saudi Arabia’s.

The excavation of the tar sands — projected to pump $2.4 trillion into Canada’s economy between 2010 and 2030 — could reduce the region’s migratory-bird population by almost half, according to a peer-reviewed study released Dec. 2 by U.S. and Canadian environmental groups.

The Connecticut warbler and the blackpoll warbler are among about 300 species affected by tar sands mining. The study estimates that over 30 to 50 years, tar sands excavation will reduce bird populations by anywhere from 6 million to 166 million, including several endangered and threatened species. The world’s only natural breeding ground for endangered whooping cranes, for example, lies north of the Albertan tar sands, and the Athabasca River, which feeds the cranes’ wetland habitat, flows north through the sands.

The report calls for a moratorium on new tar sands development pending further study of environmental impacts or, failing that, measures that include noise reduction and habitat restoration.

The mining of bitumen, a form of crude oil, from the gooey oil sands destroys habitat, drying up and contaminating wetlands where birds nest or rest during migration. Birds also land on tailing ponds, the large reservoirs where toxic runoff is stored, and often sink after becoming covered in oily residue.

“They see what looks like this great lake to spend the night on, and it turns out to be a death trap,” said Doug Stotz, senior conservation ecologist at the Field Museum in Chicago, where the study was released.

David Collyer, president of the Canadian Association of Petroleum Producers, said tar sands projects already undergo an environmental assessment process that includes effects on birds.

“There clearly is some environmental impact with any oil sands activity,” he said. “But that has to be balanced with the economic impact of these projects. The report is clearly an advocacy piece by the environmental groups. I think it misrepresents and exaggerates the environmental impacts.”

Henry Henderson, Midwest director for the Natural Resources Defense Council, one of the environmental groups involved in the study, said U.S. and Canadian endangered species laws and a migratory-birds treaty should be given more consideration when Canadian government bodies evaluate tar sands development, and when U.S. state and federal agencies judge applications for refineries and pipelines to process and transport the oil.

Henderson said the laws and the treaty “aren’t enforced at all,” partly because quantifying the wide-ranging effects of tar sands development is difficult without more data on bird populations.

Five Midwestern refineries are seeking permission to modify their plants to process Albertan tar sands oil, and Hyperion, a Dallas-based company, is proposing a new tar sands oil refinery in South Dakota.